1. Field of the Invention
This invention relates to a method of measuring blood pressure and an automatic sphygmomanometer for implementing this method. More particularly, the invention relates to a method of measuring blood pressure and an automatic sphygmomanometer in which an appropriate value of pressurization is capable of being detected automat ically.
2. Prior Art
A well-known method of measuring blood pressure includes the oscillometric method in which oscillation (referred to as a "pulse wave") produced when blood flows through a blood vessel is detected and the value of blood pressure is determined by a change in the state of the pulse wave. In accordance with this method, it is necessary to know the amplitude of the pulse wave, the change thereof and the characteristics of the waveform. Therefore, when blood pressure is measured, first the internal pressure of a cuff wound upon the patient's arm is elevated to a value 30 to 40 mmHg higher than that of the patient's systolic blood pressure or maximum blood pressure, then the cuff pressure is gradually reduced to obtain a waveform, of the kind shown in FIG. 7, indicating the change in the pulse wave with the passage of time. As shown in FIG. 7, the amplitude of the pulse wave gradually increases with respect to time, but then its amplitude decreases after it reaches a certain point. It is known that cuff pressure at the time of maximum amplitude of the pulse wave is the mean blood pressure.
Thus, when blood pressure is measured in this manner, the interior of the cuff is required to be pressurized so that its internal pressure exceeds the patient's systolic blood pressure by 30 to 40 mmHg. For this reason, the conventional sphygmomanometer is provided with a switch so that different values of increased pressure (e.g., 140, 170, 200, 240 mmHg) can be set, and the patient makes use of the switch to select and set a suitable value of increased pressure depending upon the patient's own systolic blood pressure value.
However, the prior art described above has certain disadvantages. For example, if the patient does not have a rough idea of his or her own blood pressure, or if the value of pressurization selected is inappropriate, the pressure of the cuff may be inadequate or, conversely, the pressure of the cuff may be too high. In addition, the switch provided on the sphygmomanometer for setting different values of pressurization is capable of setting these values in stages only. For example, if the values can only be set in the manner described earlier, then a patient whose systolic blood pressure is 120 mmHg must pressurize the pressure of the cuff to 170 mmHg. Pressurization to this extent can cause the patient discomfort.
In an effort to solve the foregoing problems, Japanese Patent Application Laid-Open No. 60-40038 discloses a technique in which the mean blood-pressure value is obtained during increase of the internal pressure of the cuff or inflation of the cuff and an appropriate value of pressurization is found based upon the mean value obtained. In this case, however, it is required that the rate at which the internal pressure of the cuff is raised be held at 5 mmHg/sec in consideration of the patient's pulse rate (60.about.70 beats/min), and in order to obtain the mean blood-pressure value in accurate fashion. As a consequence, an extended period of time is required to measure blood pressure. This places a considerable burden upon the patient. Furthermore, in order to obtain the mean blood-pressure value (absolute value), a calibration circuit is necessary. This inevitably results in a more complicated apparatus.